The present invention relates to motor controllers and more specifically to a method and apparatus for adaptively adjusting a motor controller as a function of a real time inertia estimate.
General-purpose industrial motor drive manufacturers supply standard drives for a, large variety of applications such as fans, pumps, conveyors, and web lines. A typical drive includes, among other components, a comparator, a load velocity sensor, a proportional-integral (PI) regulator and a current regulator. The comparator receives a feedback signal from the sensor and a reference command signal (e.g., a command velocity signal) and generates a difference or error signal. The PI regulator receives the error signal and steps up that value, as the label implies, proportionally and integrally, to generate a regulated value. The regulated value is provided to the current regulator that generates a motor torque command signal for driving an associated motor/load.
Because loads and performance requirements are different for each application, typically, a standard drive has to be “tuned” for a specific application to achieve desired results (i.e., proportional and integral gain factors have to be set as a function of a specific motor (i.e., the “plant”) and load driven by the drive). To properly tune a drive, ideally, system parameters such as plant inertia, friction, damping, and load must be known. During an off-line commissioning procedure (e.g., a procedure typically performed prior to normal operation of a drive), system parameters can be determined and used to tune the drive.
As known in the industry, at least some system parameters can vary over time and with different operating conditions and consequently it is difficult to keep a drive running optimally even if it is initially tuned off-line. In at least some applications inertia may vary over time.
One solution for dealing with changing operating parameters has been to develop model reference adaptive controllers (MRACs) that automatically tune a drive to follow a desired or model behavior. A block diagram of an exemplary MRAC 10 is shown in FIG. 1 and includes a controller 12, a reference model module 14, a plant 16, a summer 18 and an adaptive module 20. In FIG. 1, a reference command r is provided to reference model 14, controller 12 and adaptive module 20. A controller output u drives plant 16 to produce a plant output. A plant output signal x is sent back to controller 12 for closed loop control. A reference model output signal xm is subtracted from the plant output x to form an error signal ε. The reference model output signal xm and the error signal ε are also received by adaptive module 20 which calculates new controller gain(s) K in an attempt to force the closed loop system to behave like model 14. Some known systems have used the reference command signal r to calculate gain K changes. Other known schemes have used the model output signal xm in place of reference command signal r to calculate gain K changes.
Unfortunately, prior art techniques that use the reference command signal r or the model output signal xm to calculate gain K changes drastically change the structure of the simple PI regulator control loop that has been implemented in many industrial drives.